Flat display devices are increasingly being developed to replace cathode ray tube (CRT) devices. Among these, liquid crystal display devices (LCDs), organic electroluminescent displays (ELDs) and the like have particularly come to attention, which have advantages such as light weight, thin film properties, or low power consumption, etc.
For example, a light transmission active matrix type liquid crystal display device may include an array of display pixels with each having a switching element. The active matrix type liquid crystal display device has a liquid crystal layer disposed between an array substrate and an opposite substrate through an alignment layer.
The array substrate has multiple signal lines and scanning lines aligned in lattices formed on a transparent board made of glass, quartz, etc. Each of the lattices is connected to a thin film transistor (hereinafter, abbreviated to TFT) using amorphous silicon semiconductor thin films at crossing points of the lattices.
Gate electrodes and drain electrodes of the TFT are electrically connected to the scanning lines and the signal lines, respectively, while source electrodes of the TFT are electronically connected to transparent conductive materials for use in fabrication of pixel electrodes, for example, indium-tin-oxide (ITO).
Possible structures of TFTs include positive stagger type (top gate type) and negative stagger type (bottom gate type) structures.
Photoresists and photoresist films are utilized in the manufacture of these LCDs and are also used to manufacture other highly integrated semiconductors such as integrated circuits (ICs), printed circuit boards (PCBs) and electronic display devices such as cathode ray tubes (CRTs), and organic electroluminescent displays (ELs or ELDs). The manufacturing processes for these devices use photolithography and photofabrication techniques. The photoresist films require a resolution sufficient to form a pattern with extremely fine lines and small space area not more than 7 μm.
The physical properties of photoresists can vary in such characteristics as solubility in a certain solvents, coloration, curing and the like, via chemical modification of the molecular structure of the photoresist resin or the photoresist.
In recent years, processes for manufacturing TFT-LCDs using the liquid photoresist compositions have become increasingly complicated and difficult as substrate sizes are increasing, and the problems associated with liquid photoresist compositions have become more marked. Positive liquid photoresists exhibit problems such as reduced resolution and sensitivity due to sedimentation during storage, inferior pattern design due to residues on a coated surface, etc. Therefore, there exists a need to develop novel photoresists to solve such problems.
The desire for positive dry resist technology arose from the disadvantages associated with conventional liquid positive photoresists. These disadvantages led to elevated process costs. For example, spin coating a photoresist onto a semiconductor wafer results in losses of expensive photoresist material. The machinery for spin coating resists represents a substantial capital expense, and the time and management associated with spin coating results in additional process expense. The filtration associated with point-of-use application of photoresists is also cost-intensive. The wastage of photoresists at all points in the spin coating process also represents a substantial part of the photoresist cost. Also, positive liquid photoresist compositions generate insoluble materials (that is, undergoes sedimentation) during storage, leading to reduction of resolution and sensitivity. As a result, a practical dry film positive photoresist technology becomes highly desirable.
Conventional dry film photoresist technology began development during the 1960's when liquid negative photoresists were adapted to dry film technology for the manufacture of large featured, low resolution devices such as printed circuit board (PCB) patterns. However, the poor resolution of these negative dry film resists inhibited the application of dry film technology to high resolution applications such as ICs, LCDs etc.
Positive dry film resists first emerged during the 1980's, where technologies developed that exploited the properties of thermoplastic resins. For example, cellulose resins were utilized as the basis of dry film positive resists (U.S. Pat. No. 5,981,135). Additional dry film positive resists were developed by DuPont (U.S. Pat. Nos. 4,193,797 and 5,077,174), which were based upon acrylate or methacrylate resins. These related art thermoplastic positive dry film photoresists thus shared the disadvantages of the negative resists because utilizing cellulosic or acrylic resins yield a thick dry film photoresist that has low resolution.
As a result, application of these related art dry film positive photoresists has proven problematic in regards to the thin films required for advanced semiconductor manufacturing applications. That is, as the photoresist layer widths necessarily become thinner for high resolution photolithography, the requirement for a uniform thin film increases. For example, a thin film of photoresist is more sensitive to external phenomena such as substrate roughness. A sufficiently non-uniform substrate can cause defects in the photoresist layer such as “fish eye”.
Also, the physical properties of the photoresist resin or the photoresist can be altered, such as alteration in solubility in a certain solvent (that is, increase or decrease in solubility), coloration, curing and the like, via chemical modification of the molecular structure of the photoresist resin or the photoresist caused in a short time by an optical device.
Additionally, a variety of solvents used to improve physical properties and working stability of a photoresist resin composition have been developed and include, for example, ethyleneglycol monoethylether acetate (EGMEA), propyleneglycol monoethylether acetate (PGMEA), ethyl acetate (EA) and the like.
However, these liquid photoresist compositions generate insoluble materials (that is, undergoes sedimentation) during storage, leading to reduction of resolution and sensitivity. For example, a composition comprising alkali soluble novolac resin and, as a photoacid generator, a material containing 1,2-naphthoquinonediazido-4-sulfonic ester and acid decomposable radicals as disclosed in Japanese Patent Laid-Open No. 3-249654, and a composition comprising alkali soluble novolac resin, 1,2-naphthoquinonediazido-4-sulfonic polyhydroxybenzophenone ester and acid decomposable radicals as disclosed in Japanese Patent Laid-Open No. 6-202320 have problems such as reduced resolution and sensitivity due to sedimentation during storage, inferior pattern design due to residues on a coated surface, etc.
Another Example of the related art technology includes U.S. Pat. No. 3,666,473, which pertains to the use of a mixture of two kinds of phenol-formaldehyde novolac resins and a typical photosensitive compound. U.S. Pat. No. 4,115,128 discusses the addition of an organic acid cyclic anhydride to phenol resin and a naphthoquinone diazide sensitizer to improve photosensitizing speed thereof. U.S. Pat. No. 4,550,069 discusses the use of novolac resin, an o-quinone azide photosensitive compound and propyleneglycol monoethylether acetate (PGMEA) as a solvent for the same to increase photosensitizing speed and to improve human toxicity. Japanese Patent No. 189,739 is directed to fractionation of novolac resin to increase resolution and thermal resistance.
Consequently, there is a strong need in the art for a display manufacturing technology that utilizes an improved photoresist resin product that overcomes various problems such as thickness deviation of the coating layer, poor smoothness, distortion, coagulation, foaming, coating loss and the like, which are caused during necessary processes such as spin-coating or similar process in formation of micro-patterns on LCDs, organic ELDs and the like using conventional liquid positive type photoresist compositions. The technology should concurrently exhibit high resolution, excellent line width control ability, high thermal resistance, high sensitivity, high film residual rate, high dry etching resistance and high development properties; and be applicable to micro-fine processing of LCDs, organic ELDs and the like.